The manufacturing of a high number of thermoplastic containers, in particular bottles, is a process which starting from the raw material—generally polyethylene terephtalate (PET)—allows to obtain finished containers of even particularly complex shape, which is suitable for the most varied needs of the market, and particularly lightweight and resistant even when subjected to strong pressures at ambient temperature.
The production process includes a passage of deforming a preform into the final plastic container by means of the stretching-blowing step in blow machines, which are generally provided with several blow molds, which determine the final shape of the finished containers.
Blow molding is preferred today also because it is particularly suitable for making hollow bodies having a complex shape and with several undercuts so as to meet the aesthetical needs of the market in addition to allowing the making of structural elements for reinforcing specific areas of the container, which increase the sturdiness thereof, albeit with very thin body walls. Blowing has the big advantage of also allowing the production of containers with a very large body with respect to the mouth, such as bottles and flasks. Blowing is a particularly fast and efficient production process, which is suitable for the production of containers on a large scale, such as are today's beverage bottles made of thermoplastic resins, and in particular made of PET, for which the market requires particularly high production numbers. Reduced cycle-times result in the cost of the plants being divided among several pieces, thus allowing productions to be reached which are also in the range of several tens of thousands of containers per hour in larger blowing plants.
These increased production ranges also require optimizing the blow molds, which have to ensure an optimal realization of containers having a rather complex shape. The danger in cases of this kind lies in the fact that the thermoplastic material does not adequately conform to the inner wall of the mold in certain areas during the blowing, for example to all the cavities on the inner wall of the mold, thus leaving imperfections on the body of the finished container. To ensure that the material properly adheres in all areas during the expansion thereof inside the mold, which is induced by blowing the air, series of venting holes are normally provided which communicate with the exterior of the mold to allow the air in the empty mold to be released while the container expands, thus conforming to the inner surface and occupying the whole space inside the mold. Holes of predetermined dimensions are made, which are designed to ensure the release of the quantity of air occupying the inside of the mold. If the holes have a very small diameter, there is a need to provide a greater number thereof to ensure the proper venting function, while if holes are provided having a larger diameter to reduce the number thereof, the risk exists that deformations which damage the appearance of the finished container remain on the surface of the final container.
However, the problem persists of the formation of air locks, even of small dimensions, between the inner wall of the mold and the outer wall of the expanding container, despite the use of such holes. To eliminate the air locks, there is a need to blow the container using high pressures of the blowing air. However, the use of high blowing pressures is disadvantageous, in particular in terms of the operating costs of the production plant because it requires the use of more energy.
Therefore, the need is felt to improve blow molds to prevent the risk of generating defects in the blown containers while keeping the production ranges high and the blowing pressures low.